Icarus Verilog efficiency of verinum and vpp_net pow() functions

CLOSED: fixed in Icarus GIT 66bdbb7

Modules such as

module test(y);
  output [5:0] y;
  assign y = 6'd3 ** 123456789;

take forever to compile because the verinum pow() function (in verinum.cc) is actually using a loop to evaluate the power:

for (long idx = 1 ;  idx < pow_count ;  idx += 1)
    result = result * left;

(For exponents that do not fit into a long long this would also return incorrect values, but I think no-one would wait for such a case to finish compiling..)

This should of course be instead calculated using a Power-Modulus Algorithm. See the const_pow() function in Yosys (kernel/calc.cc) for an example implementation.

The vvp_net pow() function (in vvp/vvp_net.cc) seems to implement a power-modulus algorithm but nevertheless the following code snippet hangs vvp:

module test(a, y);
  input [5:0] a;
  output [5:0] y;
  assign a = 3;
  assign y = a ** 123456789;

PS: the examples above should evaluate to

assign y = 6'b110011;

2014-01-06 Reported bug on GitHub
2014-02-16 Fixed in GIT commit 66bdbb7

← Back to VlogHammer Project Page